Southern Baptist ethicist and theologian Wayne Grudem has reversed his support of Trump and urged the Republican presidential candidate to withdraw from the election.

Grudem wrote in an article for Townhall: "I cannot commend Trump's moral character, and I strongly urge him to withdraw from the election."

Grudem's reversal follows the release on Friday of a video tape from 2005, in which Trump can be heard openly speaking about groping women and trying to seduce a married woman.

Grudem also said his turnaround was as a result of reading transcripts of Trump's past comments on Howard Stern's radio show, which he described as "obscene".

"His [Trump's] vulgar comments in 2005 about his sexual aggression and assaults against women were morally evil and revealed pride in conduct that violates God's command, 'You shall not commit adultery' (Exodus 20:14)," Grudem said.

"I have now read transcripts of some of his obscene interviews with Howard Stern, and they turned my stomach. His conduct was hateful in God's eyes and I urge him to repent and call out to God for forgiveness, and to seek forgiveness from those he harmed. God intends that men honor and respect women, not abuse them as sexual objects."

Grudem said he wished he had investigated allegations about Trump earlier. In an article that has now been removed, the theologian previously wrote for Townhall that voting for Trump was a "morally good choice".

"There is no morally good presidential candidate in this election," Grudem said on Sunday.

"I previously called Donald Trump a 'good candidate with flaws' and a 'flawed candidate' but I now regret that I did not more strongly condemn his moral character."

However, Grudem also criticised Trump's rival Hillary Clinton, who he said was "no better" a candidate, and said he "absolutely" would not vote for her.

He said he was "deeply reluctant to simply walk away from the process in disgust, or vote for a write-in candidate in order to register a protest against both parties. If all the Christians in the country decide not to vote for either candidate, our rulers will then be chosen entirely by non-Christians, many of whom will increasingly use the immense power of government to promote evil, silence Christians, and oppose Christian values in every area of life. This is the opposite of what Paul told us to pray for in 1 Timothy 2:2.

"As to how I will vote, I honestly don't know at this point," Grudem concluded.

"The election is still a month away. I have friends on both sides who are surprised that I do not find this an easy question to decide. But I do not find it an easy question. I continue to pray and seek God's wisdom, and ask that God will yet provide a better solution."